Hi, See this example, which doesn't compile for now: #include <list> #include <string> #include <boost/tuple/tuple.hpp> #include <boost/iterator/zip_iterator.hpp> #include <boost/iterator/transform_iterator.hpp> int to_value(int const &v) { return v; } int main() { std::list<int> rng1; std::string rng2; boost::make_zip_iterator( boost::make_tuple( boost::make_transform_iterator(rng1.begin(), &to_value), // BidirectionalInput rng2.begin() // RandomAccess ) ); } This incurs an error: boost::detail::error_not_related_by_convertibility<T1,T2>' : base class undefined with T1=boost::detail::iterator_category_with_traversal<std::input_iterator_tag,boost::bidirectional_traversal_tag>, T2=boost::random_access_traversal_tag in minimum_traversal_category_in_iterator_tuple. template<typename IteratorTuple> struct minimum_traversal_category_in_iterator_tuple { typedef typename tuple_impl_specific::tuple_meta_transform< IteratorTuple , iterator_traversal<> // !!!!! >::type tuple_of_traversal_tags; // ... }; Couldn't "iterator_pure_traversal<>" be used instead of `iterator_traversal<>` here? The iterator_pure_traversal<> could be something like this: template< class Iterator = mpl::_1 > struct iterator_pure_traversal : boost::detail::pure_traversal_tag< typename boost::iterator_traversal<Iterator>::type > { }; Once Category is turned into pure one, the above example compiles. Is there any pitfalls if Category is "purified"? Regards, -- Shunsuke Sogame